A 53-year-old Japanese woman, without any specific medical or family history, was admitted to our hospital for acute renal failure with macrohematuria. Routine blood analysis, including blood coagulation test, revealed azotemia accompanied by prolonged activated partial thromboplastin time (aPTT). Computed tomography revealed bilateral kidney swelling with dilatation of the renal pelvis. An extensive coagulation analysis revealed that the concentration of factor VIII had decreased to 1.8% and the level of factor VIII inhibitor was markedly elevated to 19 BU/ml. The final diagnosis was acquired hemophilia induced by autoantibodies against factor VIII, which was complicated by postrenal acute renal failure due to the obstruction of urinary tracts by renal bleeding and clots. The patient was treated with a combination of prednisolone at a dose of 50 mg/day (1 mg/kg body weight) and cyclophosphamide. The levels of factor VIII inhibitor decreased gradually, and the activity of factor VIII was improved after treatment. The levels of aPTT and concentrations of factor VIII and factor VIII inhibitor were monitored during the subsequent follow-ups.